The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A refrigeration system may include one or more compressors that compress refrigerant vapor. Refrigerant vapor from the compressors may be directed into a condenser coil where the vapor may be liquefied at high pressure. The high pressure liquid refrigerant may flow to an evaporator located in a refrigeration case after it is expanded by an expansion valve to a low pressure two-phase refrigerant. As the low pressure two-phase refrigerant flows through the evaporator, the refrigerant may absorb heat from the refrigeration case and boil off to a single phase low pressure vapor that may return to the compressors. The closed loop refrigeration process may then repeat.
The refrigeration system may include multiple compressors connected to multiple circuits. Each circuit may be a physically plumbed series of cases operating at similar pressures and temperatures. For example, in a grocery store, one set of cases within a circuit may be used for frozen food, while other sets may be used for meats or dairy.
The multiple compressors may be piped together in parallel using suction and discharge gas headers to form a compressor rack. The compressor rack may include fixed capacity compressors. Suction pressure for the compressor rack may be controlled by modulating each of the fixed capacity compressors on and off in a controlled fashion. Additionally, the compressor rack may include a variable-capacity or variable-speed compressors. In such case, suction pressure for the compressor rack may be controlled by varying the capacity or speed of any variable compressor. Suction pressure may be controlled according to a suction pressure set-point. The suction pressure set-point for the rack may generally be set to meet the demand, or load, of the connected evaporator circuits.
Traditionally, suction pressure control may be accomplished by using a PID algorithm or a fuzzy logic algorithm. In both cases, suction pressure control is tuned for a specific behavior of the system load. Upon installation, a refrigeration technician expert must perform the tuning to best coordinate the control algorithm with the anticipated load. Such tuning adds to the installation cost and time. Additionally, because refrigeration system loads are constantly changing, it is difficult to accurately forecast system behavior at installation.
Traditional systems routinely overshoot the target suction pressure resulting in inefficient operation and excessive cycling of refrigeration system components. With PID control, for example, the “I” error often accumulates while a change in load has occurred. The control may then adjust capacity to return to the targeted suction pressure. Inefficient operation and excessive cycling of system components results in increased expense from wasted energy and additional maintenance.